a trend failure can now only exist in the tail number in question. That is, it may be considered a trend if it occurs more than once in a tail number, but not if it occurs once in every aircraft in the fleet.
On a single tail number, a single failure is an event, a second failure may indicate a trend but a third failure is a repetitive defect and in civil aviation is subjected to special measures. Three similar events across a fleet is a Reliability Issue and reliablity analysis is involved. The "trend Line" for the fleet is a time-related matter. Three events in three months would give a steep trend line: three events in two years would not change the slope.